Re: INSERT ... ON CONFLICT UPDATE/IGNORE 4.0

On 04/27/2015 07:02 PM, Peter Geoghegan wrote:
> So, this can still happen, but is now happening less often than
> before, I believe. On a 16 core server, with continual 128 client
> jjanes_upsert exclusion constraint only runs, with fsync=off, I
> started at this time:
>
> 2015-04-27 21:22:28 UTC [ 0 ]: LOG: database system was shut down at
> 2015-04-27 21:22:25 UTC
> 2015-04-27 21:22:28 UTC [ 0 ]: LOG: database system is ready to
> accept connections
> 2015-04-27 22:47:20 UTC [ 0 ]: LOG: autovacuum launcher started
> 2015-04-27 22:47:21 UTC [ 0 ]: LOG: autovacuum launcher started
>
> Finally, with ON CONFLICT UPDATE (which we don't intend to support
> with exclusion constraints anyway), the torture testing finally
> produces a deadlock several hours later (due to having "livelock
> insurance" [1]):
>
> 2015-04-28 00:22:06 UTC [ 0 ]: LOG: autovacuum launcher started
> 2015-04-28 00:37:24 UTC [ 432432057 ]: ERROR: deadlock detected
> 2015-04-28 00:37:24 UTC [ 432432057 ]: DETAIL: Process 130628 waits
> for ShareLock on transaction 432432127; blocked by process 130589.
> Process 130589 waits for ShareLock on speculative token 13 of
> transaction 432432057; blocked by process 130628.
> Process 130628: insert into upsert_race_test (index, count)
> values ('7566','-1') on conflict
> update set count=TARGET.count + EXCLUDED.count
> where TARGET.index = EXCLUDED.index
> returning count
> Process 130589: insert into upsert_race_test (index, count)
> values ('7566','1') on conflict
> update set count=TARGET.count + EXCLUDED.count
> where TARGET.index = EXCLUDED.index
> returning count
> 2015-04-28 00:37:24 UTC [ 432432057 ]: HINT: See server log for query details.
> 2015-04-28 00:37:24 UTC [ 432432057 ]: CONTEXT: while checking
> exclusion constraint on tuple (3,36) in relation "upsert_race_test"
> 2015-04-28 00:37:24 UTC [ 432432057 ]: STATEMENT: insert into
> upsert_race_test (index, count) values ('7566','-1') on conflict
> update set count=TARGET.count + EXCLUDED.count
> where TARGET.index = EXCLUDED.index
> returning count
>
> ON CONFLICT UPDATE will only ever use unique indexes, and so is not affected.
>
> Given that exclusion constraints can only be used with IGNORE, and
> given that this is so hard to recreate, I'm inclined to conclude that
> it's acceptable. It's certainly way better than risking livelocks by
> not having "deadlock insurance". This is a ridiculously CPU-bound
> workload, with extreme and constant contention. I'd be surprised if
> there were any real complaints from the field in practice.
>
> Do you think that this is acceptable, Heikki?

I thought we had an ironclad scheme to prevent deadlocks like this, so
I'd like to understand why that happens.

- Heikki